Tire treads for pneumatic tires typically have running surfaces of consistent rubber properties across the face of the tread intended to be ground contacting.
In particular, the rubber composition for the running surface is often optimized for a combination of acceptable traction, acceptable rolling resistance and acceptable resistance to tread wear. It is known that providing a tread rubber composition more optimized in a direction of one of such properties usually sacrifices, at least to some degree, the value of at least one of the two remaining properties.
It is to be appreciated that sometimes the peripheral, or lateral, portions of the running surface of the tread may tend to experience a greater rate of wear than the more central portion of the tread running surface, particularly for front vehicular tires which typically experience greater wear due to increased steering and cornering maneuvers.
For this invention, it is contemplated that a tread running surface is provided as a tread cap layer composed of a central zone positioned between two lateral zones located at the peripheral portion of the tread running surface, wherein the lateral zones are of a rubber composition more optimized for resistance to abrasion, or treadwear for the tire tread, and the central zone is of a rubber composition more optimized for presenting tread traction and less optimized for resistance to abrasion, or treadwear for the tire tread.
Historically, pneumatic tires often have a rubber tread of a cap/base construction where the tread cap layer is the running surface of the tire and is typically of a lug and groove configuration. Such historical tread cap layer may, for example, be comprised of a carbon-black rich or a silica-rich rubber composition.
The tread base layer underlies the tread cap layer, is not intended to be ground-contacting, and typically is of a carbon black-rich rubber composition. The tread base layer is normally intended to provide a transition between the tread cap layer and the tire carcass. Such cap/base tire tread construction and such lug and groove configuration for a tread cap as a running surface of the tread are well known to those having skill in such art.
Historically, a tire tread has heretofore been suggested having a running surface composed of three longitudinal portions namely, two black colored lateral portions and a non-black colored central portion located between the two black portions, wherein the lateral black colored portions have wear resistant properties virtually identical to the central colored portion (EP 0 993 381 A3, FR 2765525 and WO 99/01299 patent publications).
Such tire tread running surface is contrary to the present invention. For this invention, it is desired to provide a tire tread of three significantly wide, distinct load-bearing zones, of significantly different compositions, each of which contain carbon black reinforcement and are thereby black in color.
Historically, U.S. Pat. No. 5,225,011 relates to a tire having a tread composed of a center rubber composition and side rubbers (FIG. 1) positioned directly onto a tire carcass belt without a tread base transition layer. The center rubber is required to be limited to either natural rubber or a natural rubber/styrene-butadiene rubber blend. The center rubber contains a carbon black of large iodine absorption number of at least 100 mg/g, silica and silane coupling agent and the side rubbers are required to be of a different rubber composition.
Historically, European patent publication number EP 864,446 A1 relates to a tire having a tread (FIG. 2) with a central portion (B) and side portions (A) positioned directly onto a tire carcass belt without a tread base transition layer. The side portions are carbon black rich and the central portion is silica rich, wherein the silica content of the central portion (B) is at least 20 percent higher than in the side portions (A).
For the zoned tread cap layer of this invention, by requiring the tread cap zones to be load-bearing, it is meant that each of the three distinct tread running surface tread cap zones extend radially inward from the outer surface of the tread to the underlying carbon black-rich tread base rubber composition so that all of the load on the tire is communicated by the tread cap layer zones directly to the tread base layer instead of directly to remainder of the tire carcass itself.
By requiring that each of the running surface tread zones be significantly wide, and therefore each comprising a significant portion of the tread running surface, it is intended that each respective zone more effectively transmits the load from the outer surface of the running surface of the tire directly radially inward to the supportive tread base layer. For such proposes of this invention, the central zone is intended to span from about 55 to about 80 percent of the axial width of the tread cap and the two lateral, or peripheral, zones, are intended to collectively and correspondingly span from about 20 to about to about 45 percent of the axial width of the tread cap. Such span of the tread running surface is the surface of the tread lugs of the tread cap layer intended to be ground-contacting and tread grooves included between the tread lugs.
For this invention, a central tread cap zone is provided for the running surface of the tire which may be comprised of a silica-rich, carbon black-containing rubber composition.
The central tread cap zone is positioned between two lateral, or peripheral, circumferential, rubber composition zones which contain reinforcement filler as both carbon black and precipitated silica reinforcement, one lateral zone on either side of the central zone, which are also a portion of the running surface of the tire tread.
For this invention, the rubber composition for the two lateral tread cap zones has a significantly greater resistance to abrasion, according to DIN 53516 (23° C.), than the rubber composition of the central tread cap zone and therefore a greater predictive resistance to treadwear for the respective lateral tread cap zone components of the tire tread.
For this invention, the rubber composition for said central tread cap zone is intended to have a significantly higher tan delta value at 0° C. than the rubber composition for the two respective lateral tread cap zones. A relatively higher tan delta value for a central tread cap zone rubber composition at 0° C., and at the indicated low strain, is considered herein as being predictive of a comparatively increased wet traction value for the central tread cap zone component of the tire tread.
For this invention, the rubber composition for the central tread cap zone is to have a low strain modulus G′ value, at 10 percent strain, 30° C. and 10 Hertz, which is lower than the low strain modulus G′ value of the rubber composition of the associated two lateral tread cap zone components of the tire tread. The lower G′ value for the rubber composition of the central tread cap zone is indicative of having a lower resistance to abrasion as compared to the lateral tread cap zones which should have higher G′ values which is indicative to a higher resistance to abrasion.
It is considered herein that both a balance of tire tread cap layer rubber composition properties, namely the DIN abrasion, tan delta and low strain modulus properties in combination of allocating such rubber compositions between a central tread cap zone and the two associated lateral tread cap zones for the running surface of the tire, provides a tire tread with a relatively beneficial combination of comparative enhanced resistance to treadwear for the lateral tread cap zones and comparative enhanced traction for the central tread cap zone, particularly for use as vehicular front wheel driven vehicular tires.
Therefore, a purpose of such tread cap zone configuration is to provide a running surface for a tire composed of the three circumferential load bearing zones in which the rubber composition for the central tread cap zone is intended to provide relatively enhanced traction, or skid resistance, for the tire tread and the rubber composition for the two associated tread cap lateral zones is intended to provide relatively enhanced resistance to abrasion, or treadwear for the tire. Such purpose is based upon an observation, other things being equal, that the lateral portions of a tire tread running surface often tend to wear at a greater rate than its central portion and, also, that resistance to skidding of the tire is often primarily focused upon, or relied upon, by the central portion of the running surface of the tread. Accordingly, it is an aspect of this invention to provide a suitable balance of rubber compositions between tire tread cap central and lateral zones in a manner of a departure from past practice.
The rubber compositions of the central and lateral tread cap zones are comprised of different combinations of elastomers. In particular, the lateral tread cap zones are comprised of elastomers of a combination of restrictive glass transition temperatures (Tg's) which is different than the overall elastomer Tg restrictions for the elastomers of the central cap zone.
The tread cap lateral zone rubber composition is required to contain both precipitated silica and rubber reinforcing carbon black reinforcement. The central tread cap zone is required to be silica rich by containing both precipitated silica and rubber reinforcing carbon black reinforcement silica with the silica content being greater than the carbon black content.
In the description of this invention, the terms “rubber” and “elastomer” where herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise provided. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided.
In the description of this invention, the glass transition temperature (Tg) may be determined according to differential scanning calorimeter (DSC) analysis at a heating rate of 10° C. per minute.